A vector image, compared with a raster image with the same content as it has, has the feature of being independent of resolutions, and has advantages of being easy to edit and of higher compression ratio, etc. Currently, the study on raster image vectorization is preliminary, only that on binary image and engineering image vectorization is mature, and it is still a challenging problem to vectorize a general image. Gradient meshes, as a way of vectorized representation of an image, can be provided by software like Corel Draw and Adobe Illustrator. Generally, obtaining gradient meshes requires a large number of user interactions. The US patent application of Sun, Jian et al. (which application number is PCT/US2008/062970) proposed a method based on nonlinear optimization. However, the method requires user interactions to give original gradient meshes with a relatively low speed